Transmetalation for Flexible Pendant-Armed Schiff-Base Macrocyclic Complexes Influenced by Halide Effects.

Three 46-membered [2 + 2] pendant-armed Schiff-base macrocyclic dinuclear Cd and Cu complexes (2a, 2b, and 3b) and one 23-membered [1 + 1] Cu macrocycle 4a were prepared from the template-directed condensation reactions between 1,2-bis(2-aminoethoxy)-ethane and extended Cl-dialdehyde in the presence of CdX and CuX (X = Cl and Br), in which halide effects play important roles in the organization of final macrocyclic complexes in addition to the dominant influence of metal cations. Transmetalation was intensively studied among these Cd and Cu complexes with large and flexible macrocyclic ligands, including two previously synthesized dinuclear Zn macrocycles (1a and 1b). Our results indicate that Zn → Cu and Cd → Cu transmetalation proceeds more quickly than that from Cd to Zn, and all the processes are found to be irreversible. It is noted that a [2 + 2] heterodinuclear CdZn macrocyclic intermediate could be detected by ESI-MS together with [2 + 2] homodinuclear Cd and Zn macrocyclic complexes. Furthermore, distinct halide behavior was observed in the in situ Cd → Cu and Zn → Cu metal-ion exchange. That is to say, [2 + 2] macrocycles (1a and 2a) could be converted to [1 + 1] macrocycles 4a and 4b under the reflux condition in the case of CuCl, accompanied by the configurational transformation from half-folded dinuclear Zn and Cd to unfolded Cu macrocyclic skeleton. In contrast, CuBr leads to a highly efficient transmetalation to corresponding [2 + 2] dinuclear Cu complex 3b from both 1b and 2b no matter the experimental condition used.